Pontiac Trans Am SD-455 - Realizing Potential

It's difficult to grasp how an engine can generate enthusiastic buzz more than 40 years after its inception, but mention Pontiac's Super-Duty 455 and some Firebird fanatics get wide-eyed while others stand at attention in its presence. Introduced at a time when others were stepping away from performance, the SD-455 forged a mystique that became ingrained in the minds of those who read about it in vintage automobile magazines, as well as anyone who's experienced its awesome acceleration.

HPP has presented much insider information about the SD-455 and its development over the past several years. That includes comments from the Pontiac engineers directly involved with the program of how it was compromised for production. So what about the performance enthusiasts working for the Division who had no role in its development? Were they at all aware of how much potential the SD-455 possessed?

2/13

Gregg Peterson knows firsthand. A former Pontiac engineer, he arrived at PMD in 1969 and was immediately drawn toward the SD-455 program. Working closely with those associated with it on other projects, he took a keen interest in the SD-455 and gathered as much developmental information as he could. He then used it to improve the performance of his own '74 Super-Duty Trans Am. We're excited to share with HPP readers for the very first time that info and some of the internal SD-455 documents Gregg obtained.

Getting Acquainted

Gregg grew up feeling that Pontiac built America's most exciting passenger cars during the '60s, and with his love of performance automobiles he decided to make designing them a career. When it came to seeking an employer, he immediately sought out Pontiac. "I was drawn toward the Division's hell-raising attitude, the risks management was willing to take to make its cars better and more innovative, and how it was always pushing the technical limits of its talented industrial designers."

3/13On October 7, 1971, Pontiac Engineer D.L. “Skip” McCully prepared an internal memo containing several pages discussing output of the Super-Duty 455 in various configurations. The report reveals that with a compression ratio of 12:1, a high-lift aftermarket camshaft, tubular headers, and a Holley carburetor, the SD-455 produced an astounding 530 hp and 574 lb-ft!

Gregg began his work with Pontiac in 1969 as a student engineer while attending the General Motors Institute of Technology (GMI) in Flint, Michigan. The '701⁄2 Trans Am was very near its public release and upon seeing the developmental prototype for the first time, Gregg recalls, "I thought it was drop dead gorgeous. I couldn't believe a car like that was actually going into production. I always felt that a R/A-IV–powered '701⁄2 T/A was, and still is, a very serious machine."

Pontiac engineer Herb Adams headed a Special Projects group that included Skip McCully, Tom Nell, and Jeff Young. The engineering team was responsible for many programs—one of which was the SD-455. "I always talked with them," says Gregg. "They were very talented and passionate about Pontiac performance. They knew I was a performance enthusiast too, so I was asked to join Herb's NASCAR race team. Herb wanted to take Pontiac to the highest levels of performance on the street and race track and was looking to create an engine to achieve it."

Gregg first learned about the SD-455 while it was in development. "It was around 1970 and one of my student assignments was in the Powertrain group. Skip was working on a low- compression, high-performance 455 to compete with Buick's Stage 1. I believe it was this program that led to the '71 455 H.O. Jeff Young was in the Engineering lab working on a new round-port cylinder head, which turned out to be for a race-bred engine—the SD-455."

4/13In its 310hp configuration, which consisted of an 8.4:1 compression ratio and the 041 cam, this maximum power (or “GM 20”) dyno test report reveals that Pontiac observed gross output numbers in excess of 400 hp and nearly 480 lb-ft. The originals were stored on microfilm, so reproduction quality is poor.

Immediately upon learning of the SD-455 project, Gregg knew he had to own one. "Every key component was redesigned to improve performance and durability. As opposed to an improved street engine, the SD-455 was actually a detuned race engine. Management had legitimate concerns about dedicating resources to low-volume engines at a time when Pontiac was facing difficult challenges in so many areas, but there was a very high level of enthusiasm at Engineering because of the SD-455."

Ron Genaw ran Pontiac's dyno lab, and Gregg visited him on a regular basis. "Ron had built some seriously fast street Pontiacs, and in my opinion was an ideal person to be part of the Super-Duty development. In stock trim the SD-455 was capable of producing more than 400 gross hp. I recall a dyno test of a developmental SD-455 that had 12.5:1 compression, a modified Edelbrock intake manifold, a Holley four- barrel carburetor, an aggressive camshaft, and tubular headers. It put out about 600 hp and 600 lb-ft of torque. You could literally hear it running throughout the Engineering complex."

The SD-455 was planned to be an available option in several '73 Pontiac models and a number of pilot cars were built. A long-lead press preview took place at the Milford Proving Grounds on June 28, 1972, and Gregg was selected to work it. "Herb was presenting the SD-455 to the press writers that day," he adds. "He had a few different developmental Super-Duty Trans Ams on hand for performance testing, and I was assigned to run the dragstrip, which was setup on the north end of Black Lake."

5/13Herb Adams introduced the SD-455 to the world at Pontiac’s long-lead press preview on June 28, 1972. He had several cars powered by the new engine on hand for journalists to review. This particular Trans Am was an engineering mule that was fitted with a highly modified SD-455 and a four-speed manual transmission. Expect to see more information about it in an upcoming article.

Two Pontiacs from that day that stick out most in Gregg's mind are a Buccaneer Red Trans Am equipped with a near-production SD-455 and another T/A with a highly modified SD-455. "I spent time with the cars to determine the best way to launch them and fed that information to the drivers as they staged them. The SD-455 would annihilate the tires. I can remember GM Design Chief Bill Mitchell asking me how to launch the Super-Duty. The writers couldn't believe how fast the SD-455 was and that it was actually being released for production."

6/13Pontiac had a stable of Super-Duty–powered Engineering mules and one was a black ’73 Grand Prix. Gregg and Herb had just flown back to Detroit from Florida from racing the NASCAR Grand Am in the Daytona 500 qualifier, and Gregg recalls riding back to Pontiac with Herb in it. Note the NACA-scooped hood on this example, which was quite likely that very Grand Prix.

The SD-455 engine finally reached production in June 1973. Availability was, however, limited to the Firebird model line only. Though the original design saw many production compromises, which included less camshaft timing than originally intended, it was still net rated at 290 hp and 395 lb-ft and proved a stout performer. What totally remained, however, was a seemingly endless amount of potential for greater performance—and few knew that better than Gregg, who by then had graduated from GMI and was hired on by Pontiac.

Purchasing An SD-455

When the '74 Firebird was introduced, Gregg took an immediate liking to the restyle, especially the full-width rear taillights. "As a graduation present to myself I ordered a Trans Am through Pontiac Engineering when the order bank opened. Along with the SD-455, I chose an Admiralty Blue exterior with white custom interior and added performance and convenience options, including an automatic transmission, air conditioning, power windows and door locks, and Rally II wheels. I had it delivered to McMullen Pontiac in Waterford, Michigan."

Gregg was tracking his order and when he learned his Trans Am would be delivered in the middle of the winter, he immediately attempted to cancel the order. "I refused to drive it on the salted Michigan roads, if even for a short distance. I mentioned my intent to order a second car to Pontiac's production planner Tom Goad in late October and he advised to place my next order quickly or I would miss the window of availability on this limited-production engine."

On October 26, 1973, Gregg placed an order for a second identical Trans Am, which had an expected build date of early spring. While waiting for it arrive, he began thinking of ways to promote the Super-Duty's performance image to the public. Hedging on the interest generated when Car & Driver magazine pitted a new '64 GTO against the Ferrari it gleaned its name from, he proposed to Goad that Engineering develop a modified SD T/A to showcase its potential. The objective was to create a Firebird that performed and handled as well as the Ferrari Daytona for a fraction of the cost.

7/13Upon being hired on at Pontiac, engineer Gregg Peterson rewarded himself with a ’74 Trans Am powered by the SD-455. His order was initiated and approved by Pontiac Engineering on October 26, 1973. The car was delivered to McMullen Pontiac in Waterford, Michigan.

While Pontiac's marketers may have considered Gregg's proposal in preceding years, it came at a time when looming emissions regulations and economy standards dashed any hopes for the SD-455 after the '74 model year. In fact, within a few months of Gregg's proposal, Pontiac cancelled any ongoing and future developmental work on the SD-455 program. It signified the end of the SD-455 line for consumers, but Gregg anxiously awaited taking delivery of his car so the fun could begin.

Though he attempted to stop the first order from processing, it seems it may have been too far along. "For whatever reason, my first order never got cancelled because I got an unexpected call from the dealer saying my car came in. I told them I didn't want it. They probably weren't too upset though. Super-Duty Firebirds were rare and not sitting on dealer lots. They probably sold it at the sticker price or probably more."

The next Trans Am arrived in late March 1974, and though Gregg was very excited to take delivery the experience wasn't as pleasant as he had hoped. "I went to pick it up and one of the dealership mechanics told me it was the fastest Super-Duty Trans Am he had ever driven. Apparently he took these cars out for a ‘test drive' as part of the dealership's pre-delivery preparation process. Having the first 10 miles on my new car spent smoking its tires wasn't exactly how I planned to break it in!"

8/13In November 1973, Gregg created a multi-page internal report proposing that Pontiac create a press car to showcase the performance potential a Super-Duty Trans Am possessed. In addition to wider wheels and tires and new shock absorbers, his plan included modifying the SD-455 to the same specifications as a 600hp developmental mule he saw in Pontiac’s dyno lab.

Of the stock SD-455's performance in his new Trans Am, Gregg remarks, "Nothing stock is ever acceptable to me. I knew how many compromises were needed to make it a production engine and meet emissions and customer-use requirements—even a hand-built, blueprinted example like the SD-455. It certainly ran well as delivered, but I knew what it was capable of and wanted to capture some of that."

The First Modifications

Gregg was friends with many of the engineers and technicians who worked on the SD-455 program, so there was no shortage of helpful input and information available to him. "Those guys knew what worked from actual testing and I decided to incorporate some of those modifications on my own engine. I immediately replaced the Shaker's block-off plate with an open mesh screen."

9/13Gregg’s ’74 Super-Duty Trans Am was delivered in March 1974. Upon taking delivery, he mildly modified it to improve performance, and drove it sparingly for the next several years. In 1980, he extensively modified its engine and suspension to transform it into the supercar he always dreamed of.

Within its first year Gregg used a friend's chassis dyno to finely tune his SD-455's carburetor and distributor. His friend, Ed Blackerby worked in the Engineering lab and carefully assembled a centrifugal spring and weight kit that precisely tracked the unique curve Pontiac found worked best with the SD-455. "The rear wheel horsepower jumped when we installed the curve kit into the distributor and advanced the initial timing to 20 degrees BTDC (from 10)," says Gregg.

For the next few years Gregg drove his Super-Duty Trans Am in this configuration, accumulating about 12,000 miles in that time. As a single guy with some disposable income, Gregg had ordered a number of new Trans Ams in subsequent years, and even drove company cars later on, so that allowed him to keep his SD T/A a low-mile original. "I always wanted to see how much potential the SD-455 had, so I decided that if Pontiac wouldn't build my proposed Trans Am, I'd do it myself," he remarks.

10/13The first modification Gregg made upon taking delivery of his new Super-Duty Trans Am was restoring its Shaker’s functionality. A friend and coworker at Pontiac fabricated this screen and trim plate assembly for him in the engineering machine shop.

Subsequent Modifications

In 1980, Gregg and John Crawford, an experienced Pontiac technician, removed the Trans Am's front end and began disassembling the engine for significant modifications. "The plan was to increase engine output, while maintaining durability by using equipment proven on the 600hp SD-455 mule developed in the Pontiac Engineering dyno lab. I wanted to improve high-end horsepower and increase the redline by 1,000 rpm while reducing low-end torque to minimize wheel spin and improve launch characteristics."

The SD-455 was sent to Leader Automotive, where George DeLorean (brother of former Pontiac General Manager John DeLorean) prepped and machined the block. Gregg added a 4-inch-stroke, forged-steel crankshaft that was left over from the SD-421 days. "The short-stroke crank reduced displacement to 433, but it would allow the engine to operate reliably at a much higher rpm. The box it came in had ‘Smokey' written on it suggesting it was originally destined for Smokey Yunick," he adds.

11/13During development, Pontiac Engineering determined the exact amount of spark advance the SD-455 performed best with at every engine speed—a process known as mean best torque (MBT). Ed Blackerby painstakingly created a unique centrifugal advance kit for Gregg’s distributor that follows the MBT curve. It would be difficult to replicate with stock components.

While the SD-455 was apart Gregg also incorporated a special coolant design that had been developed by Louie Clements, a GM technician at the Desert Proving Grounds in Arizona. It seems that GM was using production SD-455s to conduct its high-speed tire tests and the continuous high-speed operation in very hot temperatures caused the engines to burn exhaust valves in as little as 5,000 miles.

"Louie theorized that the intake valves are sufficiently cooled by the incoming air charge," Gregg explains. "Using threaded inserts he developed a way to divert the coolant, creating a uniform flow around the exhaust valves. The modification resulted in much greater valve life and greatly reduced coolant temperature, which cured engine overheating at 110 to 115 degrees F ambient air temperatures. Louie won a major award from GM for this concept and went on to develop a similar design for the Chevrolet small-block."

Gregg obtained a print of Louie's coolant-system modification and had George DeLorean incorporate it onto his SD-455 so he could run more aggressive spark timing and a higher compression ratio together. "George machined my block and heads to match the print and the engine was able to run 20 degrees of initial advance, the aggressive distributor curve, and a compression ratio of 9.5:1 without fear of detonation or overheating."

12/13Gregg applied many of the modifications Pontiac Engineering used on its 600hp development mule to his SD-455. To make full throttle starts more manageable, he replaced the original 4.21-inch stroke with a 4.00-inch forged unit. While it reduced displacement to 433, it increased the maximum operating range by 1,000 rpm. It sports a single-plane intake and 750-cfm spread-bore carburetor. Predicted output is in excess of 500 hp.

The original SD-455 heads were port matched and all traces of casting flash removed. It was topped by a single-plane Holley Street Dominator intake manifold and 750-cfm four-barrel. Gregg replaced the original 744-spec camshaft with an aggressive 290-degree hydraulic grind capable of running to 7,000 rpm. He also added 1.65:1-ratio rocker arms, pushrods, and other valvetrain pieces from the R/A-IV.

A key part of the build was a set of 180-degree headers that John Crawford helped him create. "Herb ran them on his Trans Am race cars and the NASCAR Grand Am. I loved their distinctive high-pitch sound. At Daytona you could pick our Pontiac out of the pack by its exhaust note. I installed a set of custom-fabricated units and one of Herb's deep sump cast-aluminum oil pans on my Trans Am early on but eventually removed the headers because of ground clearance issues."

While Gregg never measured the output of his modified SD-455, his Engineering experience tells him it probably generates 500-525 gross hp. Though several horsepower shy of the 600hp dyno mule, Gregg's engine is capable of operating reliably on pump gas at 7,000 rpm, and that's the exact result he was after. He made a significant number of changes to the suspension, along with a few to the body and interior to create a Trans Am that coincides with his original proposal.

13/13The entire SD-455 program was officially cancelled on February 21, 1974, when, under management directive, Engine Project Order No. R-5501 was initiated by Skip McCully and approved by Dick Selvala. While future factory development was dead in its tracks with this document, Super-Duty lore seemingly remains as strong as ever.

Conclusion

Today Gregg's Trans Am has less than 13,000 miles on the odometer and is driven sparingly. As a rolling testament to the enthusiasm surrounding the second Super-Duty era, a look at the documents and information he provided to us illustrates the SD-455 program was something very special at Pontiac. With the enthusiasm surrounding it at the time, no wonder it's held in such high regard by enthusiasts so many years later!